The present invention generally relates to determining the Dose Area product (DAP) in an X-ray imaging system. In particular, the present invention relates to system for allowing the determination of DAP during clinical imaging without direct measurement or additional equipment.
It has long been recognized that the amount of X-ray radiation a patient is exposed to during imaging should be minimized, due to the potential ill effects possible through over exposure of the patient to X-ray radiation. In order to quantitize the amount of X-ray radiation that the patient has been exposed to, some system for measuring the dose of X-ray radiation received by the patient is needed.
Because of several factors, such as the varying contour of a patient's skin surface and even the uncertainty of the average distance from the patient to X-ray source, it is often difficult to determine the patient's actual dose of X-ray radiation. Consequently, instead of measuring the patient's actual dose of X-ray radiation, a different parameter, called the Dose Area product (DAP) is measured. The DAP is a measurement of the surface dose obtained by multiplying the dose at a given distance by the area being radiated. The DAP is consequently a measurement of the emitted X-ray radiation, rather than a measurement of the dose absorbed by the patient. However, the DAP is a good approximation of the dose the patient has received. The DAP is an important clinical tool because the DAP provides an indication of the radiation that a patient is receiving for a given procedure, which allows the doctor to monitor and adjust the dose while maintaining image quality.
The DAP thus eliminates much of the uncertainty inherent in attempting to ascertain the patient's actual skin dose. For example, the cumulative radiation dose at a surface is inversely proportional to the square of the distance between the surface and the X-ray source. Additionally, the area being imaged is directly proportional to the square of the distance between the surface and the X-ray source. Consequently, determining the DAP, the product of the dose and area, yields a parameter that is independent of source to imaging surface distance. The DAP provides an obtainable figure of merit to gauge the X-ray radiation received by a patient at any distance from the emitter.
In the past, some X-ray imaging systems have collected the necessary data for determining the DAP by inserting a measuring device in the immediate vicinity of the X-ray source and exposing the measuring device to the X-ray source during imaging. The measuring device is typically a gas-filled ionization chamber and associated electronic instrumentation and is designed to yield dose measurements or DAP measurements. Locating the ion chamber in proximity to and exposed to the X-ray source has been the only reasonable choice, since the ionization chamber must be larger than the entire X-ray source but must also not be in the way of the patient.
Because the DAP is independent of distance to the X-ray emitter, the DAP obtained by the instrumentation gives an indication of DAP anywhere along the trajectory of the X-ray beam (including at the location of the patient's skin surface). However, very close to the X-ray source, where the ion chamber is located, the X-ray beam typically includes off-axis scatter components that do not engage the patient. Additionally, the patient may generate secondary backscatter X-rays. Additional X-ray radiation from off-axis scattering or back scattered X-rays may contribute to a misleading DAP measurement. Additionally, erroneous scattering values may vary widely and in a complex way with, for example, X-ray kV settings and other conditions.
The use of the ion chamber to determine DAP during imaging is not optimal for a variety of reasons. The ion chamber and its associated instrumentation are expensive. Also, the expense of the ion chamber is multiplied because an ion chamber must be installed on each X-ray imaging system. Additionally, servicing the ion chamber may require expensive system downtime. The ion chamber and its associated instrumentation may also need to be re-calibrated often. In the case of mobile X-ray imaging systems rather than large, fixed-room systems especially, cost is of particular concern
Consequently, a need exists for a system to calculate the DAP without the additional expense of additional imaging system components. Preferably, such a system would provide for minimizing the downtime of the imaging system and provide for easy re-calibration. Also, such a system preferably minimizes additional imaging components to minimize system complexity.